Electrostatically focused traveling wave tube having periodically spaced loading elements



Li; BELOHOUBEK March 23, 1965 3,175,119

ELECTROSTATICAL FOCUSED TRAVELING WAVE TUBE HAVING PERIODICALLY SPACED LOADING ELEMENTS Filed Oct. 29, 1959 4 Sheets-Sheet 1 +33 mvzm n. 22w A Jae/mus March 23, 1965 E F. BELOHOUBEK 3,175,119

ELECTROSTATICALLY FOCUSED TRAVELING WAVE TUBE HAVING PERIODICALLY SPACED LOADING ELEMENTS Filed Oct. 29, 1959 4 Sheets-Sheet 2 INVENTOR. [PW/n fizz WWI/85K 34W, l7. MM

March 23, 1965 E. F. BELOHOUBEK 3,175,119 ELECTROSTATICALLY FOCUSED TRAVELING WAVE TUBE HAVING PERIODICALLY SPACED LOADING ELEMENTS Filed Oct. 29, 1959 4 Sheets-Sheet 4 INV EN TOR. f/PW/N fffiaawuszk Ania/7' United States Patent Office 3,175,119 Patented Mar. 23, 1965 The present invention relates to electrostatically focused traveling wave tubes.

The trend in many present day traveling wave tube applications is towards light weight, low voltage and high power. Periodic focusing of the electron beam provides means for greatly reducing the weight of the final tube assembly. Of the two basic light weight possibilities, periodic magnetic and periodic electrostatic focusing, the latter is superior especially for tubes in the lower wavelength range of the microwave spectrum, where the periodic magnetic assembly represents the major part of the total weight of the tube. Recent work on periodic electrostatic focusing with alternate apertured discs and drift tubes has shown that solid beams of high perveance can be focused successfully by the use of an optimum potential distribution between adjacent focusing electrodes. for example, a beam perveance of 2 10 a./v. can be ach eved with a voltage ratio of about 2.5 between adjacent electrodes. Besides an appreciable reduction in weight, electrostatically focused tubes also ofier two other distinct advantages. First, they are able to work over a very wide range of temperatures (the temperature compensation of magnets being restricted to a relatively narrow range). Second, by proper combination of the focusing electrodes with the RF circuit, a higher gain factor, C, can be obtained as compared to magnetically focused beams with uniform axial velocity.

Earlier work on periodic electrostatic focusing was mostly confined to bi-filar helices. Helices are, however, severly limited in their power handling capabilities and, therefore, a strong need exists for high power slow wave propagating structures which can accommodate electrostatically focused beams. Most periodic slow-wave structures, excluding structures of the helix type, comprise a fast wave conductor, such as a hollow metal waveguide, having some kind of transverse metal loading elements or partitions connected to the conductor and periodically spaced therealong in the axial direction, close to the electron beam. Such a structure can propagate waves therealong having a predominantly axial RF electric field components extending between adjacent loading elements.

The principal object of the present invention is to adapt such slow wave structures for periodic electrostatic focusing of the electron beam.

In accordance with the invention, a set of focusing elements is inserted in a periodic slow wave structure, with the focusing elements alternating with the loading elements of the structure and biased at a DC. potential different from the loading elements. Such changes affect the RF-behavior of the slow-wave structure and a number of new problems arise. For best results, the overall structure should, therefore, be made in such a way that:

(1) New modes originating from the introduction of the focusing elements are suppressed or damped sufiiciently to avoid unwanted oscillations;

(2) the RF radiation caused by the DC. isolation of adjacent elements is kept low;

(3) The interaction impedance of the working pass band is not lowered excessively; and

(4) The focusing elements are capable of handling the intercepted beam current.

By way of example only, there are disclosed herein several different slow-wave structures of the backward wave type with focusing elements inserted midway between the original cell partitions in accordance with the present invention.

It will be understood by those skilled in the art that the focusing elements are separate from and do not form a necessary part of the basic slow wave structure, even though the addition of the focusing elements to the tube within the RF field of the structure necessarily affects the RF behavior of the structure.

In the accompanying drawing:

FIGURES 1 and 2 are axial and transverse sectional views, respectively, of a slow-wave structure of the known coupled-bar type;

FIGURES 3 and 4 are axial and transverse sectional views, respectively, of the slow wave structure of FIG- URES l and 2 provided with electrostatic focusing elements in accordance with one embodiment of the invention;

FIGURES 5 through 8 are w-B graphs used in explaining the operation of the invention;

FIGURES 9 and 10 are axial and transverse sectional views, respectively, of an improved embodiment of the.

invention;

FIGURE 11 is an ]"-,8 graph forthe embodiment'of FIGURES 9 and 10;

FIGURES l2 and 13 are axial and transverse sectional views, respectively, of a complete tube embodying the invention in a folded waveguide type of slow wave structure;

FIGURE 14 is an axial transverse view of a modification of the focusing structure shown in FIGURES l2 and 13; and

FIGURE 15 is a similar view of a further modification of the focusing structure of FIGURES l2 and 13.

Slow-wave structures of the backward wave type are characterized by the fact that group velocity and phase velocity of the fundamental space harmonic are oppositely directed. Such a structure, operated as a forward wave amplifier in the first space harmonic, has a cell length nearly twice as large as a structure operating in the fundamental or zero space harmonic at the same beam voltage; it, therefore, provides enough room for the introduction of a set of focusing electrodes located midway between cell partitions. FIGURES 1 and 2 show a known slow-wave structure comprising a hollow cylindrical metal waveguide 1 with loading elements in the form of periodically-spaced parallel metal bars 3 connected at their ends to the waveguide 1 and having central beam apertures 5 with tubular extensions forming drift tubes, and FIGURES 3 and 4 show the same structure with additional focusing elements '7 inserted between the loading bars 3 for electrostatic focusing. The focusing elements 7 are connected together by a conductive strip 9. This arrangement has the advantage that the, length of the focusing period is reduced by a factor of 2 compared to a structure in which only adjacent partitions are used as focusing electrodes. Reducing the focusing period in this way results in a larger ratio of beam-hole diameter, D to focusing period length, L In the structure of FIGURE 3, L; is equal to the RF cell length L. It is well known that a wave propagated axially along the slow wave structure in FIG. 1 will establish an axial RF electric field in each RF cell as shown by the arrow B. Similarly directed RF fields will be established in each RF cell in FIG. 3. In the special case, or frequency, where [3:7r/L, the phase shift per cell is 1r radians, or and the electric fields alternate in axial direction in successive cells. Previous work on electrostatic focusing of high perveance solid beams has shown that the focusing performance improved markedly for larger ratios Qf D /L FIGURE 5 shows the wave propagation characteristic 3 of the original slow wave structure of FIGURES 1 and 2, where w=21rf, f is the wave frequency, ,6 is defined by [3,, is the phase shift per unit length for the fundamental space harmonic, and n is the number of the space harmonic plotted, where 11:0 for the fundamental space harmonic. In this graph, the 11:0 and n:l space harmonics are shown as curve A, together with the beam velocity line, v for operation as a forward wave amplifier. Any straight line through the origin in FIGURE 5, such as the line v is a constant phase velocity line, since v=w/B. The beam velocity line shown in FIG. is nearly coincident with the propagation curve A over a substantial frequency band, which means that the beam and phase velocities are nearly equal and traveling wave interaction can be obtained over this band of signal frequencies w. The qualitative determination of w-B diagrams for periodic slow wave structures is explained in detail in my article entitled Propagation Characteristics of Slow Wave Structures Derived From Coupled Resonators, published in RCA Review, vol. XIX, June 1958, pp. 2833l0. The slow wave structure of FIG- URES l and 2 herein is analyzed on pages 295-6 and 306-9 of said article.

The insertion of focusing electrodes into the basic slow-wave structure introduces several new modes. For easier identification of the various modes which are possible in the final structure, the w-p diagram for a round waveguide with the focusing electrodes alone is shown in FIGURE 6. If the focusing electrodes are short circuited to the round waveguide at the points marked X in FIGURE 4, the corresponding w-fl curves are given by the dashed lines B and C in FIGURE 6. The lower pass band (curve B) is associated essentially with the M4 resonance of the focusing electrodes while the next higher passband (curve C) belongs to the 3)\/ 4 resonance. Both passbands are fairly narrow in frequency range. If, however, the focusing electrodes are D.C. isolated from the round waveguide, as shown, the lower passband B shows a low pass characteristic, and the next higher passband C also covers a much wider frequency range than before. The upper cut-off frequencies of both passbands remain practically constant.

So far, the w-fi diagrams for the basic slow-wave structure and for the focusing elements alone in a round waveguide (without the bars present) have been discussed. A super-position of these two diagrams, FIGURE 5 and FIGURE 6, should give the w-fl diagram of the composite structure, provided that:

(1) The passband A of the coupled bar structure (FIG- URE 5) is not changed when the focusing electrodes are inserted symmetrically between adjacent bars; and

(2) The passbands associated with the focusing electrodes are not affected by the horizontal bars.

These conditions are not fulfilled entirely. For example, some additional capacitive loading normally occurs and shifts the passbands to a somewhat lower frequency. However, a good qualitative picture of the possible modes can be obtained this way. If the field configuration of the modes of both structures were perfectly orthogonal to each other (which corresponds to no coupling between those modes), the w-B diagram would be as shown in FIGURE 7. In practice, however, there is some coupling between these modes, and measured curves are of the form shown in FIGURE 8. The amount of separation between curves A and C indicates the degree of coupling. The further the curves deviate from the original crossover point, the higher is the coupling between the two modes.

Field measurements showed clearly that the lower portions of the passband A in FIGURE 8 still has the same field configuration as the original coupled-bar structure alone, while the upper end of the passband A belongs to the 3M4 resonance of the focusing electrodes (curve C in FIGURE 6). The other passband C shows a similar mode change. As can be seen from FIGURE 8, oscillation problems are to be expected from passbands B and C in forward wave amplifier applications, at the intersections of the beam velocity line v with curves A and C, besides a reduction in the useful bandwidth (as compared to FIG. 5) due to the mode crossover. Measurements indicated that the impedance of the competing passbands B and C in the range of interest is only slightly lower than the impedance of the operating passband A. However, the structure can be operated as a backward wave oscillator in the fundamental space harmonic, particularly if the drift tubes are eliminated in order to obtain a high interaction impedance of the working passband A as compared to the impedance of the passbands B and C.

In order to improve the structure shown in FIGURES 3 and 4 for forward wave amplification, various modifications involving damping or perturbing the unwanted modes without affecting the original passband of the coupled bar structure were tried, which finally led to the structure shown in FIGURES 9 and 10.

In order to better understand the properties of this structure, the structure of FIGURES 3 and 4 having the focusing electrodes 7 short circuited to the waveguide 1 at points X will be further considered. For this structure the two competing modes associated with the 7\/ 4 and 3M 4 resonance of the focusing electrodes 7 have a fairly narrow bandwith (curves B and C in FIGURE 6), and by proper shaping of the electrodes, one can obtain one passband above and one below the working passband of the coupled bar structure so that a mode crossover can be avoided. In order to provide the necessary D.C. potential for the focusing electrodes, the short at X can be replaced by separate coaxial RF choke sections. The use of chokes, however, causes the two passbands associated with the resonances of the focusing electrodes to move slightly towards each other due to the fact that the choke sections are effectively only over a certain limited frequency range. A choke below its center frequency has a slightly capacitive input impedance and, therefore, shifts the passband associated with the M4 resonance of the focusing electrodes upward in frequency. On the other hand, the passband associated with the 3M4 resonance of the focusing electrodes is lowered in frequency. A mode crossover, however, can be avoided even for very large bandwidths of the working passband if the choke sections are made with a large enough impedance ratio, 2 /2 The final structure, as shown in FIGURES 9 and 10, is obtained by rotating consecutive loading bars 3 by degrees. This increases the bandwith of the structure and decreases slightly the stored energy between bars, but does not otherwise affect the propagation characteristic of the structure. The focusing electrodes 7 are then interposed between the bars 3 at an angle of 45 degrees with respect to the bars and, therefore, lie on a helical path with a pitch equal to 4 cell lengths. This arrangement preserves the original RF periodicity and provides enough room for the placement of chokes 11 with large characteristic impedance. In addition, the two passbands asso ciated with the resonances of the focusing electrodes, which were objectionable from the standpoint of backward wave oscilaltions, could no longer be detected on cold test.

In the example shown in FIGURES 9 and 10, the waveguide 1 and loading bars 3 are formed by stacking alternately a series of relatively thick square metal blocks 12, each having a cylindrical opening 12' of large radius, and a series of thin square metal plates 13 each having two parallel elongated coupling apertures 13' one at each side of the openings 12' and forming the bars 3 therebetween.

Each of the chokes 11 comprises a low impedance quarter-wave open-ended coaxial line section formed by the combination of an aperture 1' in the waveguide 1 and a flanged metal tube 14 with a portion of a metal rod joined to one of the focusing electrodes 7. The outer portion of the rod 15 cooperates with a flanged metal cup 17 to form a high impedance quarter-wave closed-ended coaxial line section open toward the open-ended section to form a relatively Wide-band RF choke. A ceramic tube 19 is sealed between the flanges of the tube 14 and cup 17 to insulate the latter for direct currents and complete a vacuum closure for the aperture 1.

A structure of the type shown in FIGURES 3 and 4 in which all choke sections are aligned axially, showed strong damping of the passbands associated with the focusing electrodes, due to the fact that the chokes were radiating in these frequency ranges, but the two passbands could still be clearly identified. However, after the rotation of consecutive chokes by 90 degrees, as shown in FIG- URES 9 and 10, the focusing electrodes were very elfectively decoupled from each other, and careful measurements did not reveal any energy propagation in the passbands associated with the resonances of the focusing electrodes. FIGURE 11 shows the f-fi diagram of the crossed bar structure shown in FIGURES 9 and 10 designed for 5 kilowatt pulsed operation at X-band. Up to 12K mc., no other passbands besides the working passband of the coupled-bar structure could be detected. Other modes will be set up still higher in frequency, as in any other periodic slow wave structure, but the additional damping due to strong radiation of the choke sections in these frequency ranges may help appreciably to avoid oscillations. The interaction impedance of the working passband was lowered somewhat by the introduction of the focusing elements. No radiation from the choke sections was found within the frequency range of the working passband.

A disadvantage of the structure of FIGURES 9 and 10 is its somewhat complicated construction and its relatively large overall dimensions, due to the fact that the chokes 11 extend outwardly all around the basic slow wave structure. Three structures which overcome these diiliculties and, therefore, would be more suitable for applications at longer wavelengths, are shown in FIG- URES 12-15.

FIGURES 12 and 13 show a traveling wave tube having a folded-line type of slow wave structure, which is also a backward wave structure, comprising a hollow cylindrical metal waveguide 21 containing a series of periodically-spaced loading elements in the form of metal partitions 23 having central apertures in which aligned drift tubes 25 are mounted. Adjacent partitions 23 are provided with elongated coupling apertures 27 at alternate sides of the waveguide 21, as shown in FIGURE 13. In the example shown, the waveguide 21 and partitions 23 are formed by stacking together alternately a series of thick rectangular metal blocks 28 having cylindrical open lugs 28' and a series of thin rectangular metal plates 23, with the coupling apertures 27 of successive plates at opposite sides of the openings 28'.

The folded waveguide type of slow wave structure is analyzed on pages 301-3 of the RCA Review article referred to above. The electric field configuration of the fundamental mode of propagation of the folded line structure is such that it theoretically allows the introduction of focusing elements midway between the coupling apertures without coupling to them. As with a linear antenna which is inserted into the narrow side of a rectangular waveguide, no energy should be coupled out and radiated by the supporting rods of the focusing elements. In the actual structure however, small amounts of energy can still be radiated (due to fringing fields and slight asymmetries) and cause irregularities in the propagation characteristic and increase the attenuation per cell. In addition, new modes associated with resonances of the focusing electrodes are set up which have to be eliminated to avoid spurious oscillations. Several methods for minimizing these difiiculties are disclosed herein.

FIGURES 12 and 13 show the folded-line structure combined with apertured focusing elements 29 each supported between two adjacent drift tubes 25 by a rod 31 of lossy material extending radially through an opening 2i in the wall of the waveguide 21 at a point midway between successive coupling apertures 27. Each of the rods 31 extends through and is brazed to a ceramic support 33 which, in turn, is brazed at the ends to supporting surfaces on one of the blocks 28. A common D.C. focusing potential is applied to the focusing elements 29 by means of conductors 35 connected to the outer ends of the rods 31.

The rectangular blocks 28 and plates 23 are cut-away at the upper end as shown in FIGURE 13 to receive the ceramic supports 33 and provide long leakage paths therealong. The vacuum envelope for the RF portion of the tube 26 is formed by vacuum-tight brazing or soldering the stacked elements 23 and 23 together along their sides and lower ends and similarly sealing a cover plate 37 to the upper ends of the stacked elements. A potential lead 39 for the conductors 35 is brought out through the plate 3? by an insulated lead-in seal 41. The end plates 23a and 2351) are relatively thick as shown.

An electron gun, comprising a cathode d3, cathode heater 4S, focusing electrode 47 and accelerating electrode a9, is mounted on the end plate 23a, in alignment with the drift tubes 25 and the apertures in the focusing elements 29. The cathode 43 and electrode l9 are separately mounted on metal disc leads 51 and 53 which are sealed between ceramic insulator rings 55. The heater is mounted on a metal plate lead 57 sealed to the first ceramic ring 55. The last ceramic ring may be sealed to the plate 23a directly, or to an intermediate mounting plate 5'9 as shown. A cup-shaped collector 61 is sealed to the other end plate 23b in alignment with the beam axis.

FIGURE 13 shows an RF output coupling comprising a tapered rectangular waveguide section 63 extending from the last two plates 23 and 23b and the last block 28b and opening into the side of the opening 28 of the latter. Transverse plates 65 are mounted in the waveguide section 63 to provide a matching iris 67 for broad band operation. The outer end of the section 63 is sealed by a conventional closure plate 69 having an oval dielectric coupling window 71. An RF input coupling (not shown), which may be similar to the output coupling disclosed, is associated with the first block, 28a, of the waveguide 21.

The waveguide structure 21 and loading elements 23 are maintained at a given potential, say 10" kv., with respect to the cathode 43 by a DC. voltage source 73. The focusing elements 29 are maintained at a substantially higher potential, say 30 kv. by a second D.C. source 75.

In operation, the junction between each lossy supporting rod 31 and focusing elements 23, represents a strong reflection plane and most of the energy coupled into the coaxial line formed by the rod 31. and the: aperture 21; by fringing fields is reflected there. Furthermore, the small amount of energy which propagates beyond this reflection plane is attenuated in the lossy rod 31. The resistivity of the rods 31 is adjusted so as to prevent any radiation but must also be low enough to supply the necessary DC. voltage to the focusing electrodes 29, independently of the amount of intercepted beam current. In practice, values of 500 to 1000 ohms proved to be satisfactory from both standpoints. The high attenuation of the rods 31 also darnps very effectively a narrow passband which is closely associated with a N2 resonance of the focusing electrodes 29. This passband can easily be measured when the lossy supporting rods are replaced by dielectric rods. However, cold tests made on a five kilowatt X-hand model of this structure showed that the introduction of loss in the supports made it impossible to detect this mode any more. The resulting f-e diagram was nearly identical to the one for the crossed bar structure, shown in FIG. 11. This model showed a Q in the order of 1200, which corresponds to an attenuation of less than 0.05 db per cell.

The focusing electrodes 29 are preferably made of molybdenum to allow them to operate at high temperatures and, therefore, to increase the average power handling capabilities of the structure. The material for the supporting rods 31 may be a carbonized ceramic, for example. The use of a lossy material for supporting the focusing electrodes also allows the introduction of tapered attenuation which can be accomplished by expanding the lossy rods somewhat into the waveguide.

The use of lossy rods in FIGURES 12 and 13 as supports for the focusing electrodes necessitates the use of brazes between the rods and the focusing elements, and also limits the heat-dissipating capabilities and the ruggedness of the tube. An alternative structure having the same slow wave structure but using metal supporting rods 77 of varying lengths is shown in FIGURE 14. Since the lossy rods are replaced by metallic ones, new passbands occur, associated with Ira/2 resonances of the focusing electrodes 29. However, by changing the length of the supporting rods 77 at random, as shown, the periodicity of the circuit formed by the focusing electrodes 29 and their supporting rods 7'7 and connections 7 is badly distorted and no substantial energy propagation occurs in this circuit. Similiar results can be produced by using metal rods of equal length with DC. connections of random lengths between the rods.

FIGURE shows a further modification similar to the structure shown in FIGURE 14 but having metal supporting rods 81 of equal length extending through a connecting bar or strip 83 of lossy material. The lossy strip 83 heavily damps the nA/Z resonances of the focusing electrodes, and therefore, minimizes the possibility of oscillations in modes associated with resonances of the focusing structure.

Another problem that arises is the manner in which the propagation characteristic of individual cells is affected by the supporting metal rods of random lengths or by the DC. connections of random lengths. Coldtest measurements showed that as long as the focusing elements do not have an n)\/ 2 resonance within the workpassband of the structure, the propagation characteristic is not much affected when the length of the supporting rods is changed, when D.C. connections are attached to the rods or when lossy material is associated with the ends of the rods. In fact, the changes in the propagation characteristic are of the same order or even smaller than the changes which occur between different cells caused by machining and brazing inaccuracies.

The attenuation per cell was investigated by measuring the Q of an X-band folded waveguide structure which was short circuited at both ends. The Q values ranged from 1000 to 1300 which indicates that there is no appreciable loss due to radiation. In order to obtain these values, however, it is essential to align the focusing electrodes very accurately with respect to the cell partitions. Any tilting of the focusing elements, for example, leads to radiation of RF energy and a decrease in Q. The f-fl diagram for the operating pass band for each tube of FIGURES 14 and 15 is similar to the one shown in FIG- URE 11 for the crosed-bar structure of FIGURES 9 and 10. The above remarks about the reduction in interaction impedance due to the insertion of the focusing elements in FIGURES 9 and 10 apply as well here.

In tubes having focusing drift tubes of constant inner diameter, as shown in FIGS. 3, 9 and 1215, satisfactory beam focusing was obtained. However, for minimum beam scalloping, the inner surfaces of the drift tubes should be suitably shaped, e.g. a double-conical shape, to establish the optimum potential distribution between adjacent focusing electrodes at the beam boundary,

S as disclosed and claimed in a copending application of F. A. Vaccaro and W. W. Siekanowicz, Serial No. 780,783, filed December 16, 1958, now Patent No. 2,986,672, dated May 30, 1961.

What is claimed is:

1. A traveling wave tube including means consisting essentially of a hollow waveguide and a series of loading elements connected to and periodically spaced along the interior thereof for propagating a slow wave therealong, means for producing and directing an electron beam along a path adjacent to said loading elements for interaction with said wave and a series of electrostatic focusing elements one of which is interposed between the elements of each pair of adjacent loading elements.

2. A traveling wave tube including means consisting essentially of a hollow waveguide and a series of transversely extending loading elements connected to and periodically spaced along the interior thereof for propagating therealong a slow wave having predominantly RF axial electric field components extending between adjacent loading elements, means for producing and directing an electron beam along a path adjacent to said loading elements for interaction with said electric field components, and a series of transversely extending focusing elements one of which is interposed between the elements of each pair of adjacent loading elements, the ele ments of each of said series being electrically connected together for direct currents and electrically insulated for direct currents from the elements of the other series, whereby said beam can be electrostatically focused by maintaining said two series at different direct current potentials.

3. A traveling Wave tube as in claim 2, wherein said loading elements extend across the interior of said waveguide to form a capacitively coupled bar type of delay line.

4. A traveling wave tube as in claim 2, wherein each of said focusing elements is supported from said waveguide by an elongated member extending in insulated relation through an opening in said waveguide.

5. A traveling wave tube as in claim 4, including means for reducing the effect of the presence of said focusing elements and supporting members on the wave propagation characteristic of said wave propagating means.

6. A traveling wave tube as in claim 5, wherein each of said elongated members comprises attenuating material for minimizing radiation of wave energy through said openings.

7. A traveling wave tube as in claim 5, wherein said elongated members comprise conductive rods extending into a member of lossy material.

8. A traveling wave tube including means comprising a hollow waveguide and a series of transversely extending loading elements connected to and periodically spaced along the interior thereof for propagating therealong a slow wave having predominantly RF axial electric field components extending between adjacent loading elements, means for producing and directing an electron beam along a path adjacent to said loading elements for interaction with said electric field components, and a series of transversely extending focusing elements one of which is interposed between the elements of each pair of adjacent loading elements, the elements of each of said series being electrically connected together for direct currents and electrically insulated for direct currents from the elements of the other series, whereby said beam can be electrostatically focused by maintaining said two series at different direct current potentials, adjacent ones of said loading elements being provided with elongated coupling apertures alternating on opposite sides of said waveguide to form a folded-waveguide type of delay line.

9. A traveling wave tube including means comprising a hollow Waveguide and a series of transversely extending loading elements connected to and periodically spaced along the interior thereof for propagating therealong a slow wave having predominantly RF axial electric field components extending between adjacent loading elements, means for producing and directing an electron beam along a path adjacent to said loading elements for interaction with said electric field components, and a series of trans versely extending focusing elements one of which is interposed between the elements of each pair of adjacent loading elements, the elements of each of said series being electrically connected together for direct currents and electrically insulated for direct currents from the elements of the other series, whereby said beam can be electrostatically focused by maintaining said two series at different direct current potentials, the DC. connections between said focusing elements being of random lengths.

10. A traveling wave tube including means comprising a hollow waveguide and a series of transversely extending loading elements connected to and periodically spaced along the interior thereof for propagating therealong a slow wave having predominantly RF axial electric field components extending between adjacent loading elements, means for producing and directing an electron beam along a path adjacent to said loading elements for interaction with said electric field components, and a series of transversely extending focusing elements one of which is interposed between the elements of each pair of-adjacent loading elements, the elements of each of said series being electrically connected together for direct currents and electrically insulated for direct currents from the elements of the other series, whereby said beam can be electrostatically focused by maintaining said two series at different direct current potentials, said focusing elements being supported from said waveguide by elongated conductive rods of random lengths extending in insulated relation through openings in said waveguide.

11. A traveling wave tube including means consisting essentially of a hollow cylindrical waveguide and a series of apertured loading elements connected to and periodically spaced along the interior thereof for propagating therealong a slow wave having predominantly axial RF electric field components extending between adjacent loading elements, the apertures of said loading elements being coaxial with said waveguide, means for producing and directing an electron beam coaxially through said apertures of said loading elements for interaction with said electric field components, and a series of focusing elements each interposed between the elements of a pair of adjacent loading elements and having an aperture coaxial with the aperture thereof, the elements of each of said series being electrically connected together for direct currents and electrically insulated for direct currents from the elements of the other series, whereby said beam can be electrostatically focused by maintaining said two series at different direct current potentials.

12. A traveling wave tube as in claim 11, wherein said loading elements extend across the interior of said waveguide.

13. A traveling wave tube as in claim 12, wherein adjacent loading elements lie in two axial planes at right angles to each other.

14. A traveling wave tube as in claim 13, wherein each of said focusing elements is carried by an elongated conductor extending radially through an opening in said waveguide and mounted on said waveguide by an insulated choke means.

15. A traveling wave tube as in claim 14, wherein each of said elongated conductors with associated choke means lies in an axial plane at an angle of 45 to said planes of said loading elements, with the planes of successive conductors rotated by 90.

16. A traveling wave tube as in claim 15, wherein each of said choke means comprises a quarter-wave open coaxial line opening into a quarter-wave closed coaxial line, and a ceramic insulating cylinder interposed between said line.

17. A traveling wave tube as in claim 16, wherein the center frequency of said choke means is such that the passband of said loading elements is substantially midway between the passbands of the quarter-wave and threequarter-wave resonances of said focusing electrodes.

18. A traveling wave tube as in claim 12, wherein all of said loading elements lie in the same axial plane.

19. A traveling wave tube including means comprising a hollow cylindrical waveguide and a series of apertured loading elements connected to and periodically spaced along the interior thereof for propagating therealong a slow Wave having predominantly axial RF electric field components extending between adjacent loading elements, the apertures of said loading elements being coaxial with said waveguide, means for producing and directing an electron beam coaxially through said apertures of said loading elements for interaction with said electric field components, and a series of focusing elements each interposed between the elements of a pair of adjacent loading elements and having an aperture coaxial with the aperture thereof, the elements of each of said series being electrically connected together for direct currents and electrically insulated for direct currents from the elements of the other series, whereby said beam can be electrostatically focused by maintaining said two series at different direct current potentials, said loading elements comprising transverse partitions provided with elongated coupling apertures alternately on opposite sides of said waveguide to form a folded waveguide.

20. A traveling wave tube as in claim 19, wherein each of said focusing elements is supported by an elongated conductor located midway between successive coupling apertures.

21. A traveling wave tube including means consisting essentially of an elongated fast wave conductor and a series of loading elements connected to and periodically spaced along said conductor for propagating a slow wave therealong, means for producing and directing an electron beam along a path adjacent to said loading elements for interaction with said wave and a series of electrostatic focusing elements one of which is interposed between the elements of each pair of adjacent loading elements.

22. A traveling wave tube including means consisting essentially of an elongated fast wave conductor and a series of transversely extending loading elements connected to and periodically spaced along said conductor for propagating therealong a slow wave having predominantly axial electric field components extending between adjacent loading elements, means for producing and directing an electron beam along a path adjacent to said loading elements for interaction with said electric field components, and a series of transversely extending focus ing elements one of which is interposed between the elements of each pair of adjacent loading elements, the elements of each of said series being electrically connected together for direct currents and electrically insulated for direct currents from the elements of the other series, whereby said beam can be electrostatically focused by maintaining said two series at different direct current potentials.

References Cited in the file of this patent UNITED STATES PATENTS 2,809,320 Adler Oct. 8, 1957 2,832,001 Adler Apr. 22, 1958 2,843,776 Tien July 15, 1958 2,843,793 Ashkin luly 15, 1958 2,845,571 Kazan July 29, 1958 2,858,472 Karp Oct. 28, 1958 2,894,170 Rich July 7, 1959 2,924,738 Chodorow Feb. 9, 1960 2,971,113 Nygard Feb. 7, 1961 2,986,672 Vaccaro et al May 30, 1961 

21. A TRAVELING WAVE TUBE INCLUDING MEANS CONSISTING ESSENTIALLY OF AN ELONGATED FAST WAVE CONDUCTOR AND A SERIES OF LOADING ELEMENTS CONNECTED TO AND PERIODICALLY SPACED ALONG SAID CONDUCTOR FOR PROPAGATING A SLOW WAVE THEREALONG, MEANS FOR PRODUCING AND DIRECTING AN ELECTRON BEAM ALONG A PATH ADJACENT TO SAID LOADING ELEMENTS FOR INTERACTION WITH SAID WAVE AND A SERIES OF ELECTRO- 